1. Field of the Invention
The invention concerns asynchronous time-division multiplex communication medium virtual circuit throughput measuring devices.
2. Description of the Prior Art
An asynchronous time-division multiplex transmission medium conveys data messages in digital data structures called cells. Each cell comprises a header made up of five characters each of eight bits, for example, and a message body comprising a defined number of characters (48 characters, for example). These cells are transmitted continuously.
The header of each message cell identifies the destination to which the message body must be transmitted for use at the receiving end and in the form of two characters, for example. The other characters of the header contain service data. The same destination data is included in all the irregularly spaced cells constituting the same call.
The destination data thus identifies what might be called a "virtual circuit" allocated to the call and which occupies part of the transmission capacity of the transmission medium. More generally, the virtual circuit occupies part of the capacity of the transmission medium expressed as a particular throughput measured in cells per unit time, for example. This throughput fluctuates relative to a nominal throughput assigned to the virtual circuit because of characteristics of the message source and/or because of characteristics of transmission equipments the signals have previously passed through. It is therefore necessary to measure this fluctuating throughput.
The transmission medium supports at all times a plurality of virtual circuits whose cells are interleaved in an irregular manner in what is called by convention asynchronous time-division multiplexing. The nominal throughputs of the various virtual circuits are different. Each fluctuates and the sum of the fluctuating throughputs is limited by the maximum throughput of the transmission medium.
The number of virtual circuits that can be separately identified is very large, as high as 64K for example. Only a reduced number of virtual circuits are active at any time.
The transmission medium supplies the cells that it conveys to switching and transmission equipment at its downstream end. This equipment also receives cells from other transmission media. To avoid congestion of this equipment, it is necessary to check that no virtual circuit is deliberately (i.e. fraudulently) or inadvertently generating for long periods a throughput exceeding the nominal throughput assigned to it. If this occurs, corrective action is taken to prevent the transmission medium forwarding any cell regarded as exceeding the nominal throughput of the virtual circuit.
Commonly assigned U.S. Pat. No. 5,138,607 (which is hereby incorporated by reference) discloses a method of evaluating the throughput of virtual circuits for the purposes of measuring the throughput of virtual circuits and limiting this throughput to a maximum value is based on the use of a store in which each virtual circuit is assigned a location containing a set of data (called the "context") defining the throughput evaluation conditions for the virtual circuit and containing data produced by earlier application of this evaluation process. This method uses a clock adapted to provide a current time expressed in a unit associated with the virtual circuit. When each cell is received the context of the respective virtual circuit is read. This context contains a start time which is the current time as observed and recorded on receiving an earlier cell of the virtual circuit in question. This start time is subtracted from the current time and the difference obtained, together with the number of cells observed between that which caused the start time to be recorded and the present cell, provides the basis for evaluating the throughput: it is possible to deduce from this data either the number of cells in a given time interval, by waiting for the time difference to reach this time interval, or the time required to receive a given number of cells, by waiting until this number of cells is reached.
In either case, the data on which the throughput evaluation is based is a time difference determined from a current time, which is a numerical indication supplied by a clock, and a start time, which is a similar numerical indication supplied by the clock previously and then stored in the context. This time difference is established when a cell is received.
If the number of bits in the indication supplied by the clock is limited, which is particularly beneficial as this makes it possible to reduce the size of the context of each virtual circuit, it can happen that in the event of prolonged interruption of transmission of cells on a particular virtual circuit the clock for this virtual circuit completes one or more full cycles before another cell of this virtual circuit is received. The time difference then obtained is no longer indicative of the gap between the new cell and the earlier cell which caused the start time to be recorded. For this reason there is additional provision for cyclic scanning of the contexts during which, within each context, a scanning cycle counter which has an initial position is incremented. Additional means read this scanning cycle counter when a cell is received, note its position relative to its initial position and, if the noted relative position represents a given number of scanning cycles greater than 1, inhibits any corrective action taken against the cells received.
This solution is described in French patent application No 90 09 441 corresponding to commonly assigned U.S. application Ser. No. 07/732,531, now U.S. Pat. No. 5, which is hereby incorporated by reference.
When each cell is received the scan counter of the respective virtual circuit is reset to its initial position (to zero, for example).
This avoids unjustified elimination or marking of cells received after what might be termed a long silence, that is to say after one or more complete cycles of the cyclic scanning clock.
However, this device cannot detect abnormal absence of cells of a given virtual circuit. Absence of cells for a certain time indicates a break in communication if the virtual circuit is of the type guaranteeing a minimal throughput. This applies in the case of speech links, for example, where periods in which the parties are silent are encoded and cause cells having null value message bodies to be transmitted. These cells therefore maintain a certain throughput on the transmission medium. In this type of communication a long silence indicates a fault.
What is more, the prior art devices cannot distinguish between a transmission medium guaranteeing a minimal throughput and a transmission medium where absence of cells during a certain time period does not indicate a fault. It is therefore not possible to use the scan counter previously mentioned to count a period in which no cells are received.
An object of the present invention is to alleviate these drawbacks.
To be more precise, an object of the invention is to provide a device able to detect the abnormal absence of cells in an asynchronous time-division multiplex call guaranteeing a minimum throughput.